N1は N2だけです means that N1 is limited to N2(i.e., Speaking of N1, it is only N2).
Its negative equivalent is N1は N2だけでは ありません. N1 is the general category and N2 is the subcategory.
A: 学生は 日本人だけですか。
Are all the students Japanese?
B: いいえ、日本人だけでは ありません。
No, the students are not only Japanese. [There are students from other countries, too.]